Jack Horner
Applications A relatively new application with which most everyone Electroplated copper from cyanide-based plating solutions has daily contact is found in coinage, where cyanide copper has long been used both as an engineering or decorative is used to plate zinc alloy penny blanks by the millions! The finish, and as an undercoat for other plated metals. These properties of the unbrightened cyanide copper deposits are cyanide copper solutions are used to plate on a wide variety such that the coining operation produces a very bright appear- of base materials. These include steels, zinc alloys, aluminum ance. The plated penny has surprised many critics with its alloys, copper alloys, magnesium alloys, nickel alloys, and resistance to corrosion during the rigors of circulation. lead alloys. An important property of copper from cyanide- Copper is used in many other plating applications in which based systems is its ability to adhere well to these alloys. cyanide solutions are used very little. Electroforming, with a Other properties of electroplated copper deposits that are of few exceptions, is usually done from acid copper solutions. interest for various applications include: (a) soft, ductile Printed wiring boards utilize electroless copper, acid copper deposits; (b) it is easily buffed; (c) good electrical conductiv- and pyrophosphate copper, with the exception of one major ity; (d) good solderability; (e) stop-off properties on steel for manufacturer who used cyanide copper successfully on spe- selective case-hardening processes; (f) decorative finishes cial boards for many years. Copper plate is also used as a dry can be plated bright with proprietary additives, or given lubricant in some wire drawing processes, but an immersion attractive antique finishes such as oxides or patinas, (g) copper process from acid solutions is most often mentioned. copper plate from cyanide plating solutions protects acid- Immersion copper deposits are also used in coating steel sensitive base metals from attack by subsequent acid plating items such as BB shot for appearance, lubricity, and tempo- solutions; and (h) it plates easily with other metals. rary rust prevention; on welding rods for low contact resis- As an undercoat for subsequent nickel-and-chromium or tance; and on other steel parts, sometimes just for color nickel-and-brass electroplates, cyanide copper is applied in identification purposes. the decorative or functional plating of zinc alloy castings used for trim or hardware in the automotive, appliance, electrical, Processes plumbing fixtures, household goods and other industries. Copper is electroplated primarily from cyanide, acid sulfate Copper is also used as an undercoat over copper and nickel or pyrophosphate plating solutions; in the U.S., pyrophos- alloys for subsequent nickel-and-gold, silver, or nickel-and- phate is the least used. Other bath formulations have been silver electroplates in the electronic connector and electrical published or advertised but have not reached sufficient com- industries. mercial volume to be significant. Since soft copper is much easier to polish by buffing than steel, cyanide copper plated deposits can be plated on rougher Cyanide or Acid? steels and then buffed to produce “high-luster” finishes when While it is not the intent here to go into all the factors that used as an undercoat for a minimum thickness of subsequent influence the choice of which copper plating solution to use, bright nickel plating. a list of the major factors would include: As a decorative finish without subsequent electroplates of 1. Acid copper plating solutions attack most basis metals other metals, copper cyanide deposits are used on cabinet and/or produce immersion deposits with poor adhesion, hardware, hinges, drawer pulls, knobs and brackets. These especially on steel, zinc and aluminum alloy substrates. are sometimes given post-treatments to color or darken the When acid copper is to be plated over these metals, deposits, which are then highlighted by mechanical finishing cyanide copper plating is used as an undercoat to produce or selective buffing to produce antique finishes. Similar good adhesion. finishes are found on fireplace trim, lamps, lighting fixtures 2. Deposits from cyanide plating solutions exhibit much less and other household items with or without the antique post- deposit thickness variations (better throwing power) over treatment. For these applications, clear lacquers are used over the surface of a part. The more complicated the shape of a the copper plate to preserve the color and luster. part, the more significant this property becomes. Some functional applications for cyanide copper as a finish 3. Cyanide is, of course, highly poisonous and must be include plating the steel clamps on battery jumper cables, as handled with great care. Additionally, limitations on al- well as other electrical connectors and fittings, alligator clips, lowable cyanide in wastewaters are very stringent and etc. Aluminum buss bars have been plated with cyanide complete destruction methods are required in most areas. copper to provide low contact resistance for bolted connec- However, it should be recognized that cyanide destruction tions. Lead sporting ammunition is plated with heavy copper technology is well established and proven effective. deposits to prevent lead fouling of gun barrels, protect the 4. Proprietary bright acid copper solutions are capable of a projectile shape, and produce desired expansion characteris- high degree of “leveling” i.e., the smoothing of rough tics on impact. surfaces with increased plating thickness). Cyanide cop-
36 PLATING & SURFACE FINISHING per solutions generally produce the opposite effect, espe- caustic soda. A concentration of 2 to 4 oz/gal of Rochelle salts cially on rougher surfaces; however, periodic current or an equivalent proprietary additive also helps. For applica- reversal (PR) cycles and/or pulse plating with some pro- tions involving steel, 2 oz/gal caustic can be used. For use prietary additives can provide significant leveling. over zincated aluminum, omit the caustic and lower the pH to 9.8 to 10.2 with sodium bicarbonate. Strike at a current Cyanide Copper Plating Solutions Components density of 10 to 25 A/ft2 for 2 to 3 min. at 110 to 120 °F, then Copper cyanide, the source of copper in solution, is not transfer directly to the plating tank without a water rinse. soluble in water. It requires either sodium or potassium Filter continuously to avoid roughness problems. cyanide to form the soluble complexes to put it into solution. Formulations for copper plating solutions vary according Further, an excess of the alkali metal cyanide, over the to use. High speed solutions for rack plating contain 5 to 10 amount necessary to form the complexes, is needed for oz/gal copper (nominally about 6 oz/gal). Free cyanide is an sound, good quality deposits and good anode corrosion. This important component and is maintained proportionally to the excess amount is referred to as “free” cyanide or copper, usually at a 2:1 copper to “free” potassium cyanide “uncomplexed” cyanide. Caustic soda (sodium hydroxide) or ratio. Low free cyanide solutions give improved plating caustic potash (potassium hydroxide) is added to: (1) in- efficiency and higher speed; however, anode corrosion is crease conductivity, (2) reduce electrochemical attack on reduced and deposits may be rough. Caustic potash (potas- steel anode containers or any wetted exposed steel, and (3) sium hydroxide) at 2 to 4 oz/gal and Rochelle salts at 5 to 6 reduce the decomposition of cyanide. Caustic additions also oz/gal make up the remainder of the basic bath. Anodes must have an influence on anode films. Rochelle salts or propri- corrode effectively to keep roughness and cyanide oxidation etary additives are commonly used. They provide a finer- minimal. Proper anode current density range is important. If grained copper deposit, as well as improving anode corro- anodes are bagged, bag weave size is critical and the bagged sion. In addition, they limit the effects of detrimental metallic anode area should be as large as possible. impurities. For barrel plating, solutions vary in concentration from One other chemical found in cyanide copper plating solu- plant to plant. Some platers use high copper, up to 8 to 10 oz/ tions is the alkali metal carbonate. A small amount (2 to 4 oz/ gal. When barrel plating parts that nest or stick together gal) is sometimes added to new solutions to provide solution excessively, higher free cyanide can be helpful; a 1.5:1 buffering in the cathode film and improved conductivity. In copper to free cyanide ratio may be used. any case, carbonates are formed naturally in the solution with Proprietary additives are widely used in cyanide copper time, due to the decomposition of cyanide and absorption of plating solutions. These additives improve the color, yield carbon dioxide from the air. (This is a strong argument semi-bright or fully bright deposits, control the effects of against air agitation of the solution with in-plant air.) Even- various impurities, and improve anode corrosion. Brighten- tually, with no drag-out losses, or by using poorly corroding ers may be metallic, organic, or mixtures. Generally, organic copper anodes or insoluble anodes, or both, carbonates will brighteners do not produce as bright a deposit, but after build up to an excessive amount and have to be removed subsequent bright nickel plating, the deposit appears no periodically. When making the choice of whether to use different than the brighter deposits obtained from the use of potassium or sodium salts, the plater should consider that metallic brighteners. Avoid addition of metallic brighteners potassium salts allow for plating at higher current densities when plating for carburizing stop-off uses as well as for some and increase the throwing power (give more even plate oxidizing post treatments. distribution), but are more expensive than sodium salts. Additionally, because potassium carbonate is much more Post Treatments soluble than sodium carbonate, the freezing-out process for Fatigue strength is reduced by any hydrogen embrittlement removing excess carbonate is only effective with sodium that is introduced during the copper plating. To relieve salts. Chemical precipitation with calcium salts, usually lime, hydrogen embrittlement when plating on hardened steel is used for removal of excess carbonate from potassium salt- (Rockwell C35 or higher), parts are baked within 4 hr after the based solutions. final plating operation at 191±14 °C (375±25 °F) for 24 hr. Copper strike solutions are used to ensure adhesion. The When copper deposits are to be treated with a chromate strike solution is a relatively dilute, low-efficiency solution conversion coating, baking must be done before chromating. with a high cyanide-to-copper ratio, thereby producing copi- Baked parts may require a brief dip in 0.25 to 0.50 percent by ous amounts of hydrogen gas. This provides an effective gas- vol. nitric acid to remove heat oxides before chromating. scrubbing, final cleaning step while a thin layer of copper is When copper plating is used as an undercoat, the deposit may being simultaneously deposited on the part. Strike tanks are require cleaning and activating before subsequent plating, in usually smaller than the plating tank and, being good “clean- order to ensure adhesion. ers”, they act as a sump for small amounts of work-borne dirt, buffing compounds, or other contaminants that are not al- Health Impact ways removed by the prior cleaning process. In this way, Although copper is an essential element for health, excessive strike solutions protect the larger, higher metal content (and amounts can produce harmful effects. Exposure to the plated thus more expensive) cyanide plating solution. The smaller metal and exposure to the cyanide copper plating solution strike solution must be effectively filtered to reduce rough present very different situations. Copper metal in its plated deposits. If severely contaminated, the strike solution is more form presents no unusual health problems. The current OSHA economical to either treat or to destroy and dispose of. Strike permissible exposure limit (PEL) in the workplace for copper solution alkalinity will vary depending on the base metal dusts is 1 mg/m3 averaged over an 8-hr shift. Dusts could being plated. A strike solution to be used for zinc die castings, result from dry grinding of copper plated goods; however, for example, would typically contain 2 to 3 oz/gal copper, this may be an unusual possibility. 1.5 to 2.5 oz/gal free sodium cyanide, and 0.2 to 0.5 oz/gal
November 1999 37 With regard to cyanide, the OSHA PEL is 5 mg/m3 of air disposal regulations of metal-hydroxide sludges are coming averaged over an 8-hr shift. Cyanide dust is possible when into effect and require additional fixation to prevent re- handling any of the cyanide salts, while making additions or dissolution. Platers have to keep up to date in this changing making up new solutions. area. Plated parts must be rinsed thoroughly to flush out any entrapped plating solution. Any contact of cyanide with acid Trends is to be avoided, as it forms the very hazardous hydrocyanic The volume of copper plating has decreased in decorative gas (HCN). The LD50 for rats is 10 mg of cyanide per applications with the reduction in the volume of plated kilogram of body weight of the test animal. People working automotive trim. Where copper is still specified on automo- with cyanide plating solutions should be thoroughly trained tive parts, however, copper thickness requirements have been in their safe use. increased to give better protection on exterior parts. Some automotive specifications either require or suggest that a Environmental Status major portion of the copper plate thickness be from bright Wastewater limits under EPA’s Metal Finishing Regulations acid copper, and the use of cyanide copper continues to for copper are 3.38 mg/L for a single day and 2.07 mg/L as a decrease. Cyanide copper used with periodic current reversal monthly average. Total cyanide is 1.20 mg/L for a single day (PR) cycles, pulse plating, and nonmetallic, non-sulfur bright- and 0.65 mg/L for a monthly average. These are the limits eners has been used to provide equal protection with better published as compliance requirements that went into effect plate thickness distribution, but it has not found significant July 15, 1986. Specific requirements as controlled by local usage in this case to date. regulations can be much more stringent; each facility must Plating with cyanide copper will continue with probably deal with local as well as state and federal requirements. One little change in volume until a suitable, and environmentally facility discharging into surface waters in Ohio is limited to acceptable, system can replace it. The ability to plate an 23 ppb (23 µg/L) as an average with a 35 ppb daily maximum. initial coating with good adhesion to light metal base mate- A Detroit facility, discharging into the city sewer system, has rials and good throwing power and coverage in both rack and a pretreatment daily maximum 24-hr composite sample of barrel plating applications are the key features of cyanide 4.5 mg/L of copper and 2 mg/L of cyanide. copper systems that continue to justify the safety measures Copper-bearing sludges generated by platers must be dis- required in its handling and the associated cyanide destruc- posed of in an EPA-approved manner. Recent changes in tion and disposal costs. P&SF
Free Details: Circle 121 on reader service card.
38 PLATING & SURFACE FINISHING